1. Field of the Invention
This invention relates to a device for measuring vibration amplitudes of the blade tips in a turbomachine such as an aircraft turbojet or a turboprop engine for aircraft.
2. Description of the Related Art
In a known manner, a turbomachine comprises a turbine of which the rotor is driven in rotation by the flow of hot gases exiting from an annular combustion chamber arranged upstream. The rotor carries one or several wheels of blades arranged alternately with annular rows of fixed blades in a casing.
During the operation of the turbomachine, it is important to know the deformation of the mobile blades. For this purpose, it is known to mount on the casing sensors of which the sensitive element is arranged to the right of the mobile blades. The sensitive element of each sensor makes it possible to detect the passage of a blade tip (known as “tip timing”) in order to determine through comparison between the theoretical time of passage of a blade tip and the measured time of passing the deformation mode of the blade, in bending, in twisting, etc., as well as the intensity of the deformation.
The applicant has already proposed in application FR1155983 to use sensors of the capacitive type in a blower or a low-pressure compressor of a turbomachine.
However, these sensors cannot be mounted in a turbine due to the high temperature present therein and which is of a magnitude of 1000° C. In addition, in the current state of the art, these capacitive sensors have a low temporal resolution which does not make it possible to obtain deformation measurements less than 100 μm, which corresponds to a magnitude of deformations that the tips of the blades can potentially be subjected to when the latter are made of composite material having a ceramic matrix.
In order to overcome these difficulties, it has been proposed to use optical probes to detect the times of passage of the blades. Conventionally, these probes include at least two optical fibres of which one is connected to means for emitting a source of light and of which the other receives and transmits the light reflected by the blade tip to means for processing. However, these optical probes initially designed for an integration into a turbomachine compressor are poorly adapted for use in the hot environment of the turbine. It would be possible to cool them during operation by supplying for example cold air from the high-pressure compressor arranged upstream of the combustion chamber. However, such a solution would highly penalise the output and the thrust produced by the turbomachine by taking the compressed air which is used for the combustion.